Method for computer tomography and computer tomography device for carrying out the method

ABSTRACT

The invention relates to a method for computer tomography, according to which, in order to scan an object using a conical ray beam coming from a local point and a detector system for detecting the ray beam of the focal point, said detector system comprising a plurality of lines of detector elements, the focal point is displaced on a focal path about the system axis, without any relative movement between the object to be examined and the focal point, in the direction of the system axis. Said detector system supplies measuring data corresponding to the received radiation and the length of the focal path is at least the same as the length of a partial contour interval, the length of said interval being sufficient for completely reconstructing a CT image. Raw images are calculated from measuring data from a partial contour interval, the planes of said images thereof being inclined in relation to a central plane containing the local path, and a plurality of raw images are collected to form a CT image.

[0001] The invention relates to a method for computer tomography inwhich, in order to scan an object with the aid of a conical beamemanating from a focus and the aid of a detector system, having aplurality of rows of detector elements, for detecting the beam, thefocus is moved on a focal path about a system axis without relativemovement between the object to be examined and the focus in thedirection of the system axis, the detector system supplying measureddata corresponding to the received radiation, and the length of thefocal path being at least equal to the length of a partial revolutioninterval whose length suffices for the complete reconstruction of acomputer tomography (CT) image. The invention also relates to a computertomography device for carrying out such a method.

[0002] In known methods of this type, a CT image is calculated in eachcase from the measured data supplied from the individual rows. The datainconsistencies caused by the so-called cone angle (that is to say theinclination, of the rays to the image plane) occurring as a consequenceof the use of a conical beam can be neglected in this case as long asthe number of rows is sufficiently low and does not exceed 4, forexample. In the case of detector systems with a larger number of rows,for example 16 rows, however, substantial artifacts occur in the CTimages that are reconstructed on the basis of the measured data suppliedfrom the outer rows.

[0003] Methods of the Feldkamp type, in which a 3D back projection iscarried out after a convolution in the data, may be a solution to thisproblem. However, such methods are substantially more complex than 2Dreconstruction methods.

[0004] The object of the invention is to specify a method of the typementioned at the beginning that at least reduces the artifacts caused bythe use of a conical beam, doing so in a simple way. It is also theobject of the invention to specify a computer tomography device forcarrying out such a method.

[0005] This object is achieved according to the invention by means of amethod having the features of patent claim 1.

[0006] Accordingly, raw images, whose image planes are inclined relativeto a central plane containing the focal path, are calculated frommeasured data originating from a partial revolution interval. These rawimages, which can equally be CT images, are then combined to form aresulting CT image.

[0007] It thus becomes clear that in the case of the inventive methodthe image planes of the raw images are different from the image plane ofthe resulting CT image and are inclined with reference to the centralplane in contrast to the image plane of the resulting CT image. Theinclined image planes of the raw images ensure that the resulting CTimage is calculated on the basis of CT images, specifically on the basisof the raw images, that contain no artifacts, or at least no substantialones, determined by influences of the cone angle. The reason why suchartifacts cannot occur in the raw images, or can do so only to a slightextent, is that, because of the inclined image planes of the raw images,rays that run in the image plane of the respective raw image are presentfor the reconstruction of the raw images, at least over a large part ofthe respective partial revolution interval. Since the raw images thuscontain no artifacts, or at least no substantial ones, determined byinfluences of the cone angle, the pre-conditions exist for it also to bepossible to produce a resulting CT image, having few artifacts, withreference to a desired image plane by combining a plurality of rawimages. On the other hand, the radiation dose fed to an object to beexamined is used effectively since measured data originating from aplurality of rows of detector elements have some influence on theresulting CT image.

[0008] In accordance with a preferred embodiment of the invention, theraw images are combined to form a resulting CT image by weighting, thepixels of the raw images in each case contributing as source pixels to acorresponding target pixel of the resulting CT image, and thecontribution of a source pixel to a target pixel being weighted as afunction of a geometric reference variable. The result of this is thatnot only does the respectively resulting CT image have few artifactswith respect to the influences of the cone angle, but also noappreciable artifacts are produced by the combination of a plurality ofraw images. In this case, the distance of the respective source pixelfrom the corresponding target pixel and/or the distance of therespective source pixel from the center of the corresponding partialrevolution interval are/is taken into account as (a) geometric referencevariable(s).

[0009] Alternatively, the combination of a plurality of raw images toform a resulting CT image can be performed by interpolation, that is tosay the value of a pixel of the resulting CT image is determined byinterpolation from the corresponding pixels of the raw images to becombined.

[0010] In the interest of a high image quality of the resulting CTimage, it is expedient when raw images are calculated whose image planeintersect in a straight line, in particular in a tangent to the focalpath.

[0011] In cases where the aim is to achieve a particularly high temporalresolution, it is expedient to calculate raw images from a singlepartial revolution interval. Otherwise, in accordance with a preferredvariant of the invention raw images are calculated with reference to aplurality of mutually overlapping partial revolution intervals, and rawimages originating from different partial revolution intervals aresuperimposed to form the resulting CT image, since then an image qualityincreased once more is achieved in conjunction with good dose usage andlow image noise.

[0012] For each row of detector elements, the measured data comprise perposition of the focus and per detector element, a measured value that isreferred to below as a ray. As a consequence of the cone angle, the raysbelonging to the different focal positions of a partial revolutioninterval lie not only not in a common plane, but also not even in acommon surface. A particularly high image quality of the resulting CTimages is therefore achieved when, in accordance with a preferredvariant of the invention, the reconstruction of a raw image is performedon the basis of measured data that are selected from the measured data,supplied from the individual rows of detector elements, in such a waythat the rays used for the reconstruction of the respective raw imagefulfil a suitable error criterion with regard to their distance from theinclined image plane of the respective raw image.

[0013] It is ensured in this way that a raw image is respectivelycalculated on the basis of those rays that are situated in theirtotality most favorably relative to the image plane of the raw image. Asuitable error criterion is, for example, the minimum quadratic meanvalue of the distance, measured in the z direction, of all the rays,used for the reconstruction of the respective raw image, from theinclined image plane of the respective raw image.

[0014] According to variants of the invention, the layer thickness, alsodenoted as the reconstruction layer thickness, of the resulting CT imageis set via the number of raw images produced per partial revolutioninterval, or the number of raw images incorporated into the combination,and/or by weighting the raw images incorporated into the combination.

[0015] In accordance with a variant of the invention, a raw image whoseimage plane is the central plane is calculated and incorporated into thecombination. Since it is to be expected that artifacts caused by thecone angle are virtually not present in the case of such an image, theincorporation of such a raw image into the combination has a favorableeffect on the image quality of the resulting CT image.

[0016] In accordance with embodiments of the invention, it is possibleto produce, as the resulting CT image, an axial image, that is to say animage whose image plane corresponds to the central plane, a resulting CTimage with an image plane inclined to the central plane, or a resultingCT image with reference to a non-planar section of the object.

[0017] The part of the object relating to a CT device is achieved by thesubject matters of patent claims 17 to 34, and reference is made inrespect of their advantages to the above explanations of the methodaccording to the invention.

[0018] The invention is explained in more detail below with the aid ofan exemplary embodiment illustrated in the attached schematic drawings,in which:

[0019]FIG. 1 shows an illustration, in part perspective and in part as ablock diagram, of a CT device according to the invention having aplurality of rows of detector elements,

[0020]FIG. 2 shows a longitudinal section through the device inaccordance with FIG. 1 in a first operating mode,

[0021]FIG. 3 shows a further operating mode of the CT device inaccordance with FIGS. 1 and 2, in an illustration analogous to FIG. 2,and

[0022]FIG. 4 shows a further CT device in an operating mode with agreater number of active rows of detector elements than in FIGS. 2 and3, in an illustration analogous to FIG. 2.

[0023]FIGS. 1 and 2 illustrate a CT device of the third generation whichis suitable for carrying out the method according to the invention. Itsmeasuring arrangement, designated overall by 1, has an X-ray source,designated overall by 2, with a radiation aperture 3 (FIG. 2) placed infront of it and close to the source, and a detector system 5,constructed as a two-dimensional array of a plurality of rows andcolumns of detector elements—one of these is designated by 4 in FIG.1—and with a radiation aperture 6 (FIG. 2) placed in front of it andclose to the detector. In FIG. 1, for reasons of clarity, only eightrows of detector elements 4 are illustrated, but the detector system 5has further rows of detector elements 4, which is indicated by dots inFIG. 2. The X-ray source 2 with the radiation aperture 3, on the onehand, and the detector system 5 with the radiation aperture 6, on theother hand, are fitted opposite each other on a rotary frame 7, in themanner which can be seen in FIG. 2, in such a way that a pyramidal X-raybeam which, during the operation of the CT device, originates from theX-ray source 2 and is collimated by the adjustable radiation aperture 3and whose edge rays are designated by 8, strikes the detector system 5.In the process, the radiation aperture 6 is set to correspond to thecross section, set by means of the radiation aperture 3, of the X-raybeam in such a way that only that area of the detector system 5 whichcan be struck directly by the X-ray beam is exposed. In the operatingmode illustrated in FIGS. 1 and 2, this is eight rows of detectorelements 4, which are referred to as active rows below.

[0024] The further rows indicated by dots are covered by the radiationaperture 6 and are therefore inactive. Each row of detector elements 4has a number K of detector elements, k=1 to K being the so-calledchannel index. The active rows L_(n) of detector elements 4 aredesignated by L₁ to L_(N) in FIG. 2, n=1 to N being the row index.

[0025] The X-ray beam has the cone angle β, plotted in FIG. 2, which isthe opening angle of the X-ray beam in a plane containing the systemaxis Z and the focus F. The fan angle φ of the X-ray beam, which is theopening angle of the X-ray beam in a plane lying at right angles to thesystem axis Z and containing the focus F, is plotted in FIGS. 1 and 3.

[0026] The rotary frame 7 can be set rotating about a system axisdesignated by Z by means of a drive device 22. The system axis Z runsparallel to the z axis of a three-dimensional rectangular coordinatesystem illustrated in FIG. 1.

[0027] The columns of the detector system 5 likewise run in thedirection of the z axis, while the rows, whose width b is measured inthe direction of the z axis and is 1 mm, for example, run transverselywith respect to the system axis Z and the z axis.

[0028] In order to bring an object to be examined, for example apatient, into the beam path of the X-ray beam, a bearing device 9 isprovided, which can be displaced parallel to the system axis Z, that isto say in the direction of the z axis, specifically in such a way thatthere is synchronization between the rotational movement of the rotaryframe 7 and the translational movement of the bearing device, with theeffect that the ratio between translational and rotational speeds isconstant, it being possible to adjust this ratio by a desired value forthe feed h of the bearing device being selected per rotation of therotary frame.

[0029] It is therefore possible for a volume of an object to beexamined, which is located on the bearing device 9, to be examined inthe course of volume scanning, it being possible for the volume scanningto be performed in the form of spiral scanning with the effect that,with simultaneous rotation of the measuring unit 1 and translation ofthe bearing device 9, a large number of projections from variousprojection directions is recorded by means of the measuring unit perrevolution of the measuring unit 1. During the spiral scanning, thefocus F of the X-ray source is moved relative to the bearing device 9 ona spiral path designated by S in FIG. 1.

[0030] However, because there is a plurality of rows of detectorelements 4, a volume of the object to be examined can also be examinedin the course of so-called tomogram scanning in which there is norelative movement in the direction of the z axis between measuring unit1 and bearing device 9. In the case of tomogram scanning, therefore, thesize of the volume examined is determined by the number of active rowsof detector elements 4.

[0031] During tomogram scanning, the focus F moves on a circular focalpath which lies in a plane designated the central plane below. Thestraight line of intersection of the central plane with the plane of thedrawing is indicated by dashes in FIG. 2 and designated by MP, thecentral plane being at right angles to the plane of the drawing in FIG.2. A segment of the circular focal path is indicated by dots in FIG. 1and designated by CP.

[0032] The tomogram scanning can be carried out in the form of a partialrevolution or in the form of a complete revolution, the partialrevolution covering a partial revolution interval of at least π+φ, whichpermits complete reconstruction of a CT image, while a full revolutioncovers 2π.

[0033] The measured data read out in parallel from the detector elementsof each active row of the detector system 5 during the spiral ortomogram scanning and corresponding to the individual projections aresubjected to digital/analog conversion in a data conditioning unit 10,are serialized and transmitted to an image computer 11.

[0034] After the measured data have been preprocessed in a preprocessingunit 12 belonging to the image computer 11, the resulting data streampasses to a slice reconstruction unit 13, which uses the measured datato reconstruct slices of desired layers of the object to be examined. Inthe case of spiral scanning, this is performed using a method known perse (for example 180LI or 360LI interpolation), and, in the case oftomogram scanning, using a method according to the invention that is yetto be explained in detail.

[0035] The CT images are composed of pixels assembled in the form of amatrix, the pixels being associated with the respective image plane,each pixel being assigned a CT number in Hounsfield units (HU) and theindividual pixels being displayed in accordance with a CT index/grayvalue scale with a gray value corresponding to their respective CTnumber.

[0036] The images reconstructed by the slice reconstruction unit 13 andthe shadowgram reconstruction unit 15 are displayed on a display unit16, for example a monitor, connected to the image computer 11.

[0037] The X-ray source 2, for example an X-ray tube, is supplied by agenerator unit 17 with the requisite voltages and currents, for examplethe tube voltage U. In order to be able to set the latter to therespectively requisite values, the generator unit 17 is assigned acontrol unit 18 with a keyboard 19, which permits the values to be setas required.

[0038] In addition, the operation and control of the CT device apartfrom this is carried out by means of the control unit 18 and thekeyboard 19, which is illustrated by the fact that the control unit 18is connected to the image computer 11.

[0039] Amongst other things, the number N of the active rows of detectorelements 4, and therefore the position of the radiation apertures 3 and6, can be set, for which purpose the control unit 18 is connected to theadjustment units 20 and 21 assigned to the radiation apertures 3 and 6.In addition, the rotation time T can be set, which is the time needed bythe rotary frame 7 for a complete revolution and which is illustrated bythe fact that the drive unit 22 associated with the rotary frame 7 isconnected to the control unit 18.

[0040] In the case where tomogram scanning is carried out, thecalculation of the corresponding CT images is performed using a methodaccording to the invention that is explained in more detail below.

[0041] In this case, in an operating mode corresponding to a firstembodiment of the method according to the invention, tomogram scanningis carried out in the form of a full revolution (2π). Extracted from themeasured data thereby obtained is a number of measured datacorresponding to mutually overlapping partial revolution intervalsN_(α), from which in each case a number of raw images N_(tilt) arecalculated, whose pixels relate to different image planes inclined withreference to the central plane.

[0042] It may be seen from FIG. 3, in which an object to be examined,illustrated in cross section, is designated by OBJ, that, in the case ofthe exemplary embodiment described, there are four mutually overlappingpartial revolution intervals, that is to say it holds that N_(α)=4. Thepartial revolution intervals are designated in FIG. 3 by PRI₁ to PRI₄.In order to form a partial revolution interval, it may be necessary tobring together measured data from the start and end of the tomogramscanning to form a partial revolution interval.

[0043] As may be seen from FIG. 4, using the example of the partialrevolution interval PRI₄, five raw images are calculated per partialrevolution interval in the case of the exemplary embodiment described,that is to say it holds that N_(tilt)=5, which is illustrated by theimage planes PI₁ to PI₅ of the raw images. Thus, a total ofN_(α)*N_(tilt)20 raw images are calculated from the measured data of thefull revolution, and are finally combined to form a resulting CT image.

[0044] The image planes PI₁ to PI₅ of the raw images all intersect in astraight line in accordance with FIG. 4. In the case of the exemplaryembodiment illustrated, this straight line is the tangent T at thecenter M of the respective partial revolution, that is to say at thatpoint of the segment, belonging to the partial revolution interval, ofthe focal path that is situated at half the arc length of this segmentof the focal path.

[0045] For each of these image planes PI₁ to PI₅, those measured valuesthat correspond to the line integrals required for a completereconstruction of the respective raw image are now selected from themeasured data supplied from the various detector rows L₁ to L₈, theselection being performed in such a way that the rays used for thereconstruction of the respective raw image fulfil a suitable errorcriterion with regard to their distance from the inclined image plane ofthe respective raw image, this being, in the case of the exemplaryembodiment described, the minimum quadratic mean value of the distance,measured in the z direction, of all the rays, used for thereconstruction of the respective raw image, from the respective inclinedimage plane PI₁ to PI₅.

[0046] The maximum inclination of a preliminary image plane is thereforedetermined by the requirement that there must be available for all therequisite line integrals measured values whose rays are situatedsatisfactorily close to the inclined image plane in accordance with theerror criterion.

[0047] These line integrals assembled from various measured values foreach image planes PI₁ to PI₅ are now used to calculate a raw imagebelonging to the respective image plane PI₁ to PI₅, for example by meansof the standard reconstruction method of convolution and backprojection. The pixels of this raw image belong to the respectiveinclined image plane PI₁ to PI₅. Thus, a stack of five raw images iscalculated for each partial revolution interval in the case of theexemplary embodiment described.

[0048] The N_(α)*N_(tilt) raw images thus obtained are combined in asubsequent reformatting step to form a resulting CT image of a desiredimage plane that differs from the image planes PI₁ to PI₅, specificallyas a function of selectable submodes yet to be explained, either byweighting or by interpolation. Independently of the respective submode,the image noise is reduced in the course of the combination, and thedesired reconstruction layer thickness is set.

[0049] In an operating mode corresponding to a second embodiment of themethod according to the invention, instead of being calculated frommeasured data of a plurality of partial revolution intervals obtained inthe course of a full revolution, the raw images are calculated only frommeasured data of a single partial revolution interval. This operatingmode is advantageous, in particular, for applications in which the aimis to achieve as high a temporal resolution as possible, for exampleexaminations of the heart.

[0050] Whereas in the case of the first operating mode the raw imagesbelonging to a plurality of partial revolution intervals are combined toform a resulting CT image, it follows that in the case of the secondoperating mode only raw images belonging to a single partial revolutioninterval are combined to form a resulting CT image.

[0051] The combination of raw images to form a resulting CT image isperformed according to a first submode, selectable both in the first andin the second operating mode, by weighting, the procedure being such inthe case of the combination by weighting that it is performed accordingto one of two selectable weighting modes in a fashion independent of therespectively selected weighting mode in such a way that the pixels ofthe raw images in each case contribute as source pixels to acorresponding target pixel of the resulting CT image, and thecontribution of a source pixel to a target pixel is weighted as afunction of a geometric reference variable. In other words: the CTnumber belonging to a target pixel is respectively determined from theCT numbers of the corresponding source pixels taking account of thegeometric reference variable.

[0052] In the first weighting mode, the distance of the respectivesource pixel from the corresponding target pixel is taken into accountas a geometric reference variable.

[0053] In the second weighting mode, in order to avoid artifacts anadditional weighting is performed as a function of the distance of thesource pixels from the center of the respective partial revolutioninterval.

[0054] In a second submode, the combination of the raw images to form aresulting CT image is performed by interpolation, that is to say thetarget pixels, i.e. the pixels of the resulting CT image, are determinedby interpolation, for example linear interpolation, from thecorresponding source pixels, i.e. from the corresponding pixels of theraw images.

[0055] Apart from the described operating modes, submodes and weightingmodes, it is possible to select so-called slice modes that determinethose image planes for which the resulting CT image is generated.

[0056] Apart from a first slice mode in which the resulting CT image isdetermined for an image plane at right angles to the system axis, forexample the central plane MP, a second slice mode is provided in whichthe resulting CT image is determined for an image plane inclined withreference to the system axis, for example the image plane NP inaccordance with FIG. 3. For the first slice mode, it is possible to usethe keyboard to input the z position of the image plane, that is to saythe point of intersection of the image plane with the system axis Z. Forthe second slice mode, it is possible, in addition, to use the keyboardto input the angle of inclination of the image plane with reference totwo axes of the three-dimensional coordinate system illustrated in FIG.1.

[0057] It is possible in a third slice mode, for example by using alight pen 24 to draw on the monitor 16, to provide a curved section, forexample the curved section CA in accordance with FIG. 1, for which theresulting CT image is determined. The point of intersection of thecurved section CA with the system axis Z can be marked by means of thelight pen 24, and the z position [lacuna] section CA on the system axisZ can be input by means of the keyboard 19.

[0058] The spatial position of the respectively selected image planeand, in the case of a section, also the course thereof are taken intoaccount when combining the raw images to form a resulting CT image, byvirtue of the fact that, depending on the selected submode, arbitraryoblique or even curved secondary sections are also produced directlyfrom the stack of preliminary images (if appropriate, also from aplurality of stacks of different adjacent tomograms) in the weighting orinterpolation method.

[0059] If no suitable measured values, that is to say rays, areavailable for fulfilling the error criterion, measured valuescorresponding to the error criterion can be obtained from a plurality ofmeasured values situated near the image plane of the raw image, but notsufficiently near according to the error criterion, for example byadding up said measured values in conjunction with suitable weighting.

[0060] The method according to the invention also comprises thepossibility of forming a resulting CT image by superimposing raw imagesof a plurality of stacks of raw images that have been obtained on thebasis of tomogram scannings with various central planes spaced apartpreferably only slightly in the z direction.

[0061] In the case of the exemplary embodiments described, the relativemovement between the measuring unit 1 and bearing device 9 is in eachcase produced by the bearing device 9 being displaced. However, withinthe scope of the invention, there is also the possibility of leaving thebearing device 9 in a fixed position, and instead, of displacing themeasuring unit 1. In addition, within the scope of the invention, thereis the possibility of producing the necessary relative movement bydisplacing both the measuring unit 1 and the bearing device 9.

[0062] The conical X-ray beam has a rectangular cross section in thecase of the exemplary embodiment described. However, othercross-sectional geometries are also possible within the scope of theinvention.

[0063] In connection with the exemplary embodiments described above, CTdevices of the third generation are used, that is to say the X-raysource and the detector system are displaced jointly about the systemaxis during the image production. However, the invention can also beused in conjunction with CT devices of the fourth generation, in whichonly the X-ray source is displaced about the system axis and interactswith a stationary detector ring, if the detector system is a multi-rowarray of detector elements.

[0064] The method according to the invention can also be used in CTdevices of the fifth generation, that is to say CT devices in which theX-radiation does not emanate from only one focus but from a plurality offoci of one or more X-ray sources displaced about the system axis, ifthe detector system has a multi-row array of detector elements.

[0065] The CT devices used in conjunction with the exemplary embodimentsdescribed above have a detector system with detector elements arrangedin the manner of an orthogonal matrix. However, the invention can alsobe used in conjunction with CT devices whose detector system hasdetector elements arranged in a two-dimensional array in another manner.

[0066] The exemplary embodiments described above relate to the medicalapplication of the method according to the invention. However, theinvention can also be applied outside medicine, for example in luggagechecking or in material examination.

1. A method for computer tomography, having the following method steps:in order to scan an object with the aid of a conical beam emanating froma focus and the aid of a detector system, having a plurality of rows ofdetector elements, for detecting the beam, the focus is moved on a focalpath about a system axis without relative movement between the objectand the focus in the direction of the system axis, the detector systemsupplying measured data corresponding to the received radiation, and thelength of the focal path being at least equal to the length of a partialrevolution interval whose length suffices for the completereconstruction of a CT image, raw images whose image planes are inclinedrelative to a central plane containing the focal path are calculatedfrom measured data originating from a partial revolution interval, and aplurality of raw images are combined to form a resulting CT image. 2.The method as claimed in claim 1, in which the raw images are combinedby weighting, the pixels of the raw images in each case contributing assource pixels to a corresponding target pixel of the resulting CT image,and the contribution of a source pixel to a target pixel being weightedas a function of a geometric reference variable.
 3. The method asclaimed in claim 2, in which the distance of the respective source pixelfrom the corresponding target pixel is taken into account as a geometricreference variable.
 4. The method as claimed in claim 2 or 3, in whichthe distance of the respective source pixel from the center of thecorresponding partial revolution interval is taken into account as ageometric reference variable.
 5. The method as claimed in claim 1, inwhich the combination of the raw images is performed by interpolation.6. The method as claimed in one of claims 1 to 5, in which raw imagesare calculated whose image planes intersect in a straight line.
 7. Themethod as claimed in claim 6, in which the image planes of the rawimages intersect at a tangent to the focal path.
 8. The method asclaimed in one of claims 1 to 7, in which raw images are calculated withreference to a plurality of partial revolution intervals, and raw imagesoriginating from different partial revolution intervals are superimposedto form the resulting CT image.
 9. The method as claimed in claim 8, inwhich raw images are calculated with reference to a plurality ofmutually overlapping partial revolution intervals.
 10. The method asclaimed in one of claims 1 to 9, in which measured data are obtainedthat, for each row of detector elements, comprise one ray per positionof the focus and per detector element, and in which reconstruction of araw image is performed on the basis of measured data that are selectedfrom the measured data, supplied from the individual rows of detectorelements, in such a way that the rays used for the reconstruction of therespective raw image fulfil an error criterion with regard to theirdistance from the inclined image plane of the respective raw image. 11.The method as claimed in claim 10, in which the minimum quadratic meanvalue of the distance, measured in the z direction, of all the rays,used for the reconstruction of the respective raw image, from theinclined image plane of the respective raw image is provided as an errorcriterion.
 12. The method as claimed in one of claims 1 to 11, in whichthe layer thickness of the resulting CT image is set via the number ofraw images incorporated into the combination.
 13. The method as claimedin one of claims 1 to 12, in which the layer thickness of the resultingCT image is set by weighting the raw images incorporated into thecombination.
 14. The method as claimed in one of claims 1 to 13, inwhich a raw image whose image plane is the central plane is calculatedand incorporated into the combination.
 15. The method as claimed in oneof claims 1 to 14, in which an axial image is produced as the resultingCT image.
 16. The method as claimed in one of claims 1 to 14, in which aresulting CT image with an image plane inclined with respect to thecentral plane is produced.
 18. The method as claimed in one of claims 1to 16, in which the calculation of the raw images is performed by imagereconstruction, preferably on the basis of a conventional reconstructionmethod.
 19. A computer tomography device (CT device) for carrying out amethod as claimed in one of claims 1 to 18, having a radiation sourcewith a focus from which a conical beam serving for scanning an objectemanates, a detector system, having a plurality of rows of detectorelements, for detecting the beam, and a system axis about which thefocus is moved on a focal path, without a relative movement taking placebetween the object and the focus in the direction of the system axis,the detector system supplying measured data corresponding to thereceived radiation, the length of the focal path being at least equal tothe length of a partial revolution interval whose length suffices forthe complete reconstruction of a CT image, and an image computer beingprovided that uses measured data originating from a partial revolutioninterval to calculate raw images whose image planes are inclinedrelative to a central plane containing the focal path, and combines aplurality of raw images to form a resulting CT image.
 19. The CT deviceas claimed in claim 18, in which the image computer undertakes thecombination of the raw images by weighting, the pixels of the raw imagesin each case contributing as source pixels to a corresponding targetpixel of the resulting CT image, and the contribution of a source pixelto a target pixel being weighted as a function of a geometric referencevariable.
 20. The CT device as claimed in claim 19, in which the imagecomputer takes account of the distance of the respective source pixelfrom the corresponding target pixel as a geometric reference variable.21. The CT device as claimed in claim 19 or 20, in which the imagecomputer takes account of the distance of the respective source pixelfrom the center of the corresponding partial revolution interval as ageometric reference variable.
 22. The CT device as claimed in claim 18,in which the image computer undertakes the combination of the raw imagesby interpolation.
 23. The CT device as claimed in one of claims 18 to22, in which the image computer calculates raw images whose image planesintersect in a straight line.
 24. The CT device as claimed in claim 23,in which the image planes of the raw images intersect at a tangent tothe focal path.
 25. The CT device as claimed in one of claims 18 to 24,in which the image computer calculates raw images with reference to aplurality of partial revolution intervals, and superimposes raw imagesoriginating from different partial revolution intervals to form theresulting CT image.
 26. The CT device as claimed in claim 25, in whichthe image computer calculates raw images with reference to a pluralityof mutually overlapping partial revolution intervals.
 27. The CT deviceas claimed in one of claims 18 to 26, in which the detector systemobtains measured data that, for each row of detector elements, compriseone ray per position of the focus and per detector element, and in whichthe image computer undertakes the reconstruction of a raw image on thebasis of measured data which it selects from the measured data, suppliedfrom the individual rows of detector elements, in such a way that therays used for the reconstruction of the respective raw image fulfil anerror criterion with regard to their distance from the inclined imageplane of the respective raw image.
 28. The CT device as claimed in claim27, in which the image computer uses the minimum quadratic mean value ofthe distance, measured in the z direction, of all the rays, used for thereconstruction of the respective raw image, from the inclined imageplane of the respective raw image as an error criterion.
 29. The CTdevice as claimed in one of claims 18 to 28, in which the layerthickness of the resulting CT image can be set via the number of rawimages incorporated into the combination.
 30. The CT device as claimedin one of claims 18 to 29, in which the layer thickness of the resultingCT image can be set by weighting the raw images incorporated into thecombination.
 31. The CT device as claimed in one of claims 18 to 30, inwhich the image computer calculates a raw image whose image plane is thecentral plane, and incorporates it into the combination.
 32. The CTdevice as claimed in one of claims 18 to 31, in which the image computerproduces an axial image as the resulting CT image.
 33. The CT device asclaimed in one of claims 18 to 32, in which a resulting CT image with animage plane inclined with reference to the central plane is produced.34. The CT device as claimed in one of claims 18 to 33, in which theimage computer undertakes the calculation of the raw images by imagereconstruction, preferably on the basis of a conventional reconstructionmethod.